1. Field of the Invention
This invention relates, in a first aspect thereof, to a joined body of a ceramic member and a metallic member and, in a second aspect thereof, to a process for joining a ceramic member and a metallic member to each other.
The joined body according to the first aspect of this invention and the joining process according to the second aspect of the invention are applicable to not only sliding component parts such as tappets, rocker arms, valve bridges etc. in internal-combustion engines but also a variety of tools employing brazing (tools comprising a super hard material, an Si.sub.3 N.sub.4 material, etc.), vacuum switches, surge arresters, thyristors, vacuum-sealed terminals, IC packages electrode material, joined parts with a difference in expansion characteristic, etc.
2. Description of the Prior Art
Engineering ceramics such as silicon nitride have excellent mechanical strength, heat resistance and wear resistance, and they have recently been paid attention to as wear-resistant material for engine component parts or the like. However, ceramics are generally hard, brittle and poor workability. Therefore, composite structures comprising a metallic material and a ceramic material connected to each other are commonly used in many cases.
Connection of a metallic material and a ceramic material is generally carried out by thermal joining using a brazing filler material. Ceramics comprising Si.sub.3 N.sub.4, SiC or the like, however, have coefficients of expansion or contraction equivalent to 1/3 to 1/4 times those of metals. It is difficult to achieve normal joining of such a ceramic and a metal, because a strain is generated in the ceramic due to the difference in contraction coefficient between the ceramic and the metal at the time of contraction in a joining process. Therefore, joining of a ceramic member and a metallic member has hitherto been conducted with stress relaxation by disposing a Cu or other soft metal plate (shock-absorbing plate) between the ceramic member and the metallic member.
In this type of conventional joining body, the shock-absorbing plate is required in addition to a brazing filler material, in order to absorb the difference in expantion or contraction between the ceramic member and the metallic member. If the diameter of the joint is too large, the use of the shock-absorbing plate is not enough to absorb the difference in contraction. This method thus involves limitation or difficulties in application to joining of a ceramic member and a metallic member with a large joint diameter. Besides, the conventional joining method requiring the shock-absorbing plate for joining is disadvantageous on an economical basis, involves troublesome setting of members, and has a possibility of the shock-absorbing plate being left unset.
On the other hand, a direct joining method without using any shock-absorbing plate may be contemplated, in which a metallic material to be joined may be the 42 nickel alloy, Kovar or the like having a relatively low coefficient of expansion. These low-expantion metals, however, generally have an inflection point at a temperature of around 250.degree. to 450.degree. C., at which the expansion coefficient is rapidly increased. Thus, it is difficult to apply the direct joining method and the low-expansion metals to low-expansion ceramics such as silicon nitride, silicon carbide and aluminum nitride. In the case of joining such a low-expansion metal to alumina, there is the problem of high material cost because the low-expansion metal contains expensive Co or Ni in a large amount. Accordingly, there has been a keen request for the development of a directly joined body of a ceramic member and a metallic member, having little joint strain and being inexpensive, as well as a joining method for obtaining the joined body.